Technical Field
The present invention relates to superconductor qubits, and more particularly to devices and methods for tuning qubit frequencies.
Description of the Related Art
Superconducting quantum bits (qubits) show great promise for quantum computing, with ever-increasing coherence times combined with the possibility of standard semiconductor fabrication methods leading to a useful quantum computer. However, one outstanding problem is in the control of qubit frequencies.
Quantum computers will require qubits with tightly controlled distributions of frequency. For the specific case of superconducting qubits, the frequency is determined by a Josephson junction critical current Ic and a total shunt capacitance, with a frequency given by f=½π√{square root over (LC)}. The inductance L is dominated by that of the Josephson junction and is given by
  L  =            Φ      0              2      ⁢      π      ⁢                          ⁢              I        c            where Φ0 is the flux quantum. The capacitance C is the sum of junction capacitance and the shunt capacitance. Typically, experiments and estimates both show that the capacitance can be well controlled and typically fluctuations are due to variations in junction critical current. Typical distributions of frequency have standard deviations of 5-10%. It is desirable to control the frequency of the qubit population to much better than this, typically providing standard deviations of less than 1%.
The observed spread in Josephson junction critical currents is in the range of 10% standard deviation, corresponding to ˜5% spread in qubit frequency. Despite decades of research into fabrication and control of Josephson junctions, no significant improvement has been seen in the spread observed. Similar spreads are observed in other oxide junction technologies such as magnetic memory (MRAM), indicating that this may be an intrinsic property of such devices.
Qubit frequencies need to be controlled, and the fabrication process typically used for making these devices results in a spread of frequencies, which is insufficient for application in a large scale quantum computer.